Wastewater is a byproduct of many industrial processes that use water. For example, the food industry relies rather heavily on water for processing food. Water is used to clean vegetables, beef, fish, poultry, and other types of food often before the food is cooked, blanched or sterilized using other water. Unfortunately, after all this water is used it typically must also be processed to clean it so it can be either reused or inexpensively disposed. As a result of rather restrictive environmental laws and regulations that have made disposal of unprocessed wastewater prohibitively expensive, wastewater processing or preprocessing at the site of the food processing facility is desirable.
One apparatus that has proved particularly effective at treating wastewater is a cylindrical-type wastewater screen and such is shown and described in my U.S. Pat. No. 5,433,849, issued Jul. 18, 1995. As shown in that patent, a cylindrical-type wastewater screen has a cylindrical screen, typically comprised of perforate wedgewire, into which the wastewater is introduced while the screen is rotated. The wastewater passes radially outwardly through the screen after which it can be reused, further filtered, or disposed. Solids entrained in the wastewater that were filtered out of the wastewater can be cheaply disposed of as landfill or fertilizer. As shown, two cylindrical-type screens having successively finer screen media can be concentrically arranged to provide staged treatment of wastewater.
During operation, wastewater introduced within the cylindrical screen passes radially outwardly through perforations in the screen while most of the solids entrained in the wastewater is filtered by and retained in the screen because the perforations typically are no greater than about ten to twenty thousandths of an inch. The filtered solids often cling to the screen and the screen is rotated to cause gravity to encourage the solids to separate from the screen and fall to the bottom of the screen. A small flow of wastewater at the bottom of the screen carries the solids from the screen helping to keep the screen clean.
Many times, sticky solids, such as fat, connective tissue, coatings, starch, and other sticky residue will continue to cling to the screen despite rotation of the screen. The sticky solids can also cause other solids in the wastewater to stick to it and in general will significantly reduce the efficiency of the screen by partially or completely plugging perforations. Should too many perforations become plugged, the screen will have to be taken offline and cleaned.
To help keep the screen clean to prevent too many perforations from becoming plugged, fixed or stationary nozzles carried by a manifold which is disposed adjacent the screen can discharge cold water, hot water, steam or even air forcefully against the screen. Several spaced apart nozzles must be used to clean the screen along its entire axial length. Unfortunately, where water is used, each nozzle typically requires a flow of as much as about 3 gallons per minute such that a typical screen can use as much as 30 to 90 gallons of water per minute, depending on the number of nozzles required and the type of screening apparatus used. For example, where a double cylinder wastewater screen is used, such as is disclosed in U.S. Pat. No. 5,433,849 to Zittel, as many as 30 nozzles fixed to at least two manifolds are used to keep both cylinders clean. While the frequency of cleaning can vary with the type of solids entrained in the wastewater, each cleaning cycle nonetheless uses a lot of water which undesirably increases wastewater treatment costs.
While reciprocating sprayers have been used in some types of similar processing equipment, they have not been viewed as particularly well suited for use in cylindrical-type because of reliability considerations and because of concern they could not effectively clean the rather small perforations found in wastewater screens. For example, it is known to use a reciprocating sprayer to direct water against a perforate cylinder of a bean snipper to free the perforations of bean parts that have become lodged in them. However, the perforations in the cylinder of a bean snipper are much larger, typically at least 13/64 (0.203) of an inch, the cylinder is made of polycarbonate, and a bean snipper is used to cut beans, not process wastewater. Moreover, it is not believed that the reciprocating sprayer used in bean snippers discharges water at a great enough pressure to completely pass through the exterior of the screen adjacent the sprayer and impact against the interior of the screen on the opposite side.